The present invention relates to a multi-beam scanning optical system in which a plurality of light beams emitted from a plurality of light sources are deflected to scan on a surface to be scanned.
A scanning optical system to be employed in a laser printer for forming a monochrome (e.g., a black-and-white) image is provided with a laser diode, which is driven in accordance with image data. The laser beam emitted by the laser diode is collimated by a collimating lens, and is directed to a deflecting member such as a polygonal mirror. The laser beam, which is incident on light reflecting surfaces of the rotating polygonal mirror, is deflected to scan within a predetermined angular range. The scanning laser beam is incident on an fxcex8 lens, refracted and converged thereby, and then is incident on an evenly charged photoconductive surface of a photoconductive drum to form a beam spot which moves along the rotational axis of the photoconductive drum (i.e., along a main scanning direction). Since the laser diode is driven in accordance with the image data, the surface of the photoconductive drum is exposed to light corresponding to the image data. While the light beam scans on the surface of the photoconductive drum, it is rotated (i.e., an auxiliary scanning is performed). Thus, a two-dimensional latent image is formed on the photoconductive surface of the photoconductive drum.
Then, toner is applied to the latent image to form a developed image, which is transferred onto a recording sheet and fixed thereon.
Recently, color laser beam printers, which are capable of forming color images, have been developed. In the color laser printer, generally, a plurality of laser diodes are provided (which will be referred to as a multi-beam laser printer). Further, the corresponding number of fxcex8 lenses, and the corresponding number of photoconductive drums are provided for forming images of respective color components (e.g., yellow, magenta, cyan and black components). The above-described, exposing and developing processes are performed for each color component, and the images (developed images) for the four color components are transferred on a recording sheet and fixed.
In the multi-beam laser printer as described above, a plurality of laser diodes are provided. A laser diode has a characteristic such that the temperature thereof increases when driven to emit the laser beam. Further, when the temperature increases/decreases, the wavelength of the laser beam emitted by the laser diode increases/decreases. Therefore, if each of the plurality of laser diodes are driven at different timing, temperature differences occur among the laser diodes, and accordingly, the wavelengths of the laser beams emitted by the laser diodes become different from each other.
An fxcex8 lens of the scanning optical system is configured to exhibit the same optical characteristic to the light beams which have the same wavelength. However, if the wavelengths of the light beams are different from each other, lateral chromatic aberration become significant, and length of scanning lines formed by each beams may become different. If such a problem occurs in the above-described color laser beam printer, positional differences in the main scanning direction occur among each color components, which results in a color shift of the color image finally recorded on a recording sheet.
In view of the above problems, it is an object of the present invention to provide an improved scanning optical system in which the positional differences, in the main scanning direction, of the images formed by the plurality of beams having different wavelengths can be suppressed.
For the above object, according to one aspect of the invention, there is provided a multi-beam scanning optical system, which includes a light source unit emitting a plurality of light beams, the plurality of beams being parallel to each other, a polygonal mirror having a plurality of reflection surfaces arranged along a rotational direction of the polygonal mirror, the plurality of beams emitted by the light source unit being incident on the polygonal mirror and reflected by the reflection surfaces of the polygonal mirror, and an fxcex8 lens system, the plurality of beams reflected by the reflection surface of the polygonal mirror passing through the fxcex8 lens system and proceeding toward surfaces to be scanned. In such a scanning optical system, the fxcex8 lens includes a first lens, a second lens and a third lens, the plurality of beams deflected by the polygonal mirror being incident on the first, second and third lenses in this order. The first lens consists of a first lens element and second lens element, the plurality of beams being incident on the first and second lens elements in this order, the first lens element converging the plurality of beams in an auxiliary scanning direction, the second lens element converging the plurality of beams in a main scanning direction. Further, a plurality of convex surfaces corresponding to the plurality of beams are formed on one of surfaces of the first and second lens elements, the plurality of convex surfaces extending in the main scanning direction, the plurality of convex surfaces converging the plurality of beams in the auxiliary scanning direction, respectively, a diffraction lens structure for compensating for lateral chromatic aberration caused by a refraction lens structure of the fxcex8 lens system being formed on each of the plurality of convex surfaces.
With this configuration, shift of the imaging areas for each color components due to the lateral chromatic aberration of the refraction lens structure of the fxcex8 lens can be suppressed.
Optionally, the convex surfaces are formed on a light receiving surface of the first lens element.
In particular, the first lens, which includes the first lens element, converges the plurality of beams mainly in the auxiliary scanning direction.
Alternatively, the convex surfaces are formed on a light emerging surface of the second lens element.
In this case, the first lens, which includes the second lens, may converge the plurality of beams mainly in the auxiliary scanning direction.
Further optionally, the first lens element, the second lens element and the diffraction lens structure are integrally formed.
In a particular case, the second lens may converge the plurality of beams mainly in the main scanning direction.
According to another aspect of the invention, there is provided a multi-beam scanning optical system, which includes a light source unit emitting a plurality of light beams, the plurality of beams being parallel to each other, a polygonal mirror having a plurality of reflection surfaces arranged along a rotational direction of the polygonal mirror, the plurality of beams emitted by the light source unit being incident on the polygonal mirror and deflected by the reflection surfaces of the polygonal mirror, an fxcex8 lens system, the plurality of beams reflected by the reflection surface of the polygonal mirror passing through the fxcex8 lens system and proceeding toward surfaces to be scanned, and a cover member that surrounds the polygonal mirror, at least a first portion, through which the plurality of beams directed to the polygonal mirror enter, and a second portion, from which the plurality of beams reflected by the polygonal mirror emerge, are formed to be light transmitting portions. In this case, a plurality of diffraction lens structures corresponding to the plurality of beams for compensating for lateral chromatic aberration caused by a refraction lens structure of the fxcex8 lens system are formed at least on the second portion.
With this configuration, shift of the imaging areas for each color components due to the lateral chromatic aberration of the refraction lens structure of the fxcex8 lens can be suppressed.
Optionally, a through opening is formed on the cover member, and a light transmitting member fitted in the opening, the light transmitting member including the first and second portions.
In a particular case, the plurality of diffraction lens structure are formed either of the first and second portions.